Thermal switch



Patented Apr. 2l, 1936 UNITED4 STATES A 2,037,689 PATENT OFFICE THERMAL SWITCH Application November 25, 1931, Seria No. 577,340.

Renewed September 12, 1935. In Sweden December 3, 1930 4 Claims.

The present invention refers to thermal switches of the kind which are enclosed in hermetically sealed gas lled receptacles. More particularly the invention refers to such thermal switches of said kind in which the thermostat is actuated through heat supplied from the outside, for instance through a heating winding disposed externally about the receptacle. It will be understood that the invention may well be applied to switches of the character referred to in which the thermostat is actuated through thermal. changes emanating internally as Well as externally of the receptacle.

Throughout the specification and claims, the

terms thermostatic member and heat responsive member are used to signify that part of the device which operates the contacts in response to thermal changes. The development in the electric power field more and more tends towards control of even very large amounts of energy by means of thermal switches and in connection herewith one strives to attain an effective current interruption when using a small distance between the contacts and to increase the breaking speed to the greatest possible extent. By disposing the thermal switch in vacuum the magnitude of the requisite contact movement may be greatly reduced, but the obtainable breaking speed, i. e. the speed at which the thermostat will operate when its temperature exceeds a certain ambient temperature, is rather limited because in such a case the heat transfer from the surroundings is effected substantially only through radiation. When using a gas lling of view that it favours a f a suitable kind, particularly a filling consisting yof low-atomic inert gases, for instance hydrogen, the heat transfer is, however, considerably increased on account of the comparatively great heat conductivity o f such gases. The most suitable iilling is hydrogen which has the highest heat conductivity. A high heat conductivity of the gas filling s of great importance for the operation of the thermal switch also from the point rapid cooling of the glowing points of contact and of the arc set up at the opening of the contacts.

The use in thermal switches of a gas filling consisting of low-atomic gases has, however, been attended with considerable diiculties depending upon conditions hitherto not understood. Through investigations carried out in connection with the`development of the present invention it has been possible to establish that said dilliculties greatly dependV upon the occurrence of glow-discharge phenomena at the hitherto used gas pressures and at the usual operating voltages. If the gas pressure is considerably in excess of atmospheric pressure no glowing occurs at normal operating voltages, it is true, but a considerable excess of pressure cannot be used in (Cl. 20D-144) thermal switches having glass receptacles unless the receptacle walls are given quite impractical dimensions. The glow-discharge phenomenon disappears also at a strong evacuation, the heat conductivity of the gas lling then, however, de-

creasing at the same time very rapidly against zero whereby the capability of the gas filling to conduct heat from the receptacle walls to the rthermostatic member of the switch is correspondingly eliminated.

The glow discharge results, rstly, in a detrimental heating of the contacts which may cause irregularities in the' operation of the thermal switch and such deformations of the contact members that therestoration of the contacts to closed position is rendered impossible, and, secondly in using the switch in direct current circuits, in order to produce a gradually'occurring ruining of the contacts through the unidirectional action of the glow discharge and the consequent transport of electrode material from the one electrode to the other.

Theexperimental investigations on which the present invention is based have, however, further shown that there'is a certain limited evacuating range within which there exists on the one side no risk for glow-discharge phenomena whereas on the other hand the heat conductivity of the gas maintains approximately its maximum value.

The transfer of metallic particles from one electrode to another upon interrupting the circuit at a given voltage lis considerably more pronounced at atmospheric pressure than at reduced pressure. This transfer results ln the formation of small contact points which after greater or smaller number of breaks will render the cntacts inoperable. As thev number of breaks` is a measure of the longevity of a switch this question is of a great importance. As according -to the invention a pressure lower than atmospheric is used the life of the contacts is correspondingly increased.

'I'he invention will be more closely described for the case that hydrogen is used and with reference to the accompanying drawing, on which Figure 1 is a diagram indicating the variation in the glow-discharge voltage with varying hydrogen pressures and distances between the contacts, and Figure 2 shows diagrammatically how the heat conductivity of hydrogen, expressed in per cent, varies with the pressure at a. constant temperature of about 150 C. and Fig. 3 shows a hermetically sealed receptacle, as a practical embodiment of the invention.

The diagram shown in Figure 1 illustrates the connection, defined through the so called Paschens law, between on the one hand the glowvoltage V for hydrogen and, on the other hand, the product pxd of the hydrogen pressure p in millimeters and the distance d between the contacts also expressed in millimeters. At a certain definite distance d between the contacts the glow-voltage decreases with decreasing gas pressure comparatively slowly down to a certain minimum C, as will be seen from the diagram. The pressure prevailing at the point C is called the critical pressure and the pressure at which glow discharge just appears or disappears respectively at a given voltage and a varying pressure may be termed glow discharge pressure. At a further decrease in pressure the glow-voltage will increase, to begin with slowly and then more and more rapidly, following the curve branch BA The lowest glow-voltage lies somewhat above 270 volts. Glow-discharge phenomena can thus occur only if the highest voltage occurring exceeds said limit value. if it is'assumed that alternating current of 220 volts is used, the maximum voltage of which current is about 312 volts, there is evidently a critical range BD within which glow-discharge may occur at the switch contacts. Said range lies approximately between the limit values 7,5 and i8 of the product mrd which values, however, refer to contact distances the order of magnitude of which is somewhat higher than the distances which will ever occur in the operation of thermal switches. The values of pand, which in this connection limit the dangerous range, probably lie somewhat lower but this is of no importance for the investigation of the principle.

From the above it will be evident that one must avoid operation of the thermal switch within said critical range BD. On the other hand, one cannot for practical reasons have the switch operating exclusively above said range, i. e. on the curve portion DE, as this would involve the necessity of selecting too great a pressure for the gas. It is, however, conceivable to arrange the switch in such a way that the contacts during a period of short duration will -take up positions corresponding to the critical range BCD in the breaking and closing movements, the switch being then adapted to operate with so great a breaking and particularly closing speed that during the rapid passage of said dangerous range BCD the glow-discharge would not have sufiicient time to have any detrimental effect. This case is, however, of practical importance only for certain quick make and break switches which are of no interest in connection with the present invention.

At least from a theoretical point of view there is the further possibility of reducing the distance between the contacts to such an extremely small value that the product pxd will always remain below the value corresponding to B even at comparatively large pressures. From a manufacturing point of view, this would, however, even if practicable not be preferable.

'I'hus the only possibility remaining is that of selecting such a low pressure that even at dis- .tances between the electrodes Aconceivable in practice the glow-voltage will lie higher than the maximum operating voltage, thus in thel above cited case 312 volts. One then meets with the above mentioned diiculty that at very low pressures the gas iilling loses its heat conductivity and thereby its useful action.

It has, however, now been established through tests that a certain limited range for the values of the product pxd may be brought to correspond to a pressure range within which the heat conductivity of the gas still very nearly maintains its maximum value. This possibility is based upon the fact that with a rising pressure the heat conductivity of, for instance, hydrogen asymptotically approaches said maximum value and already at a comparatively low pressure reaches approximately said maximum value as will be seen from Figure 2, in which the abscissa is constituted by the gas pressure p in millimeters and the ordinate by the percentage heat conductivity, i. e. the heat conductivity A expressed in percent of the heat conductivity A at atmospheric pressure. At a falling pressure the heat conductivity begins to decrease slowly first at 8 to i0 millimeters so that at the pressure of two millimeters it still is about of the heat conductivity at atmospheric pressure. Below a pressure of l millimeter the heat conductivity decreases more and more rapidly with falling pressures and ceases completely at vacuum. lf according to the invention the pressure of the gas lling is selected within the limits thus defined the desired effect of the gas filling is fully obtained without disturbing influences on account of formation of glowlight at the point of contact.

I claim:-

l. A switch comprising a hermetically sealed receptacle, contacts disposed inside said receptacle, a thermostatic member for operating said contacts, a receptacle iilling of inert rarefied gas at a pressure less than glow discharge pressure and below critical but above that value at which the heat conductivity of the gas amounts to twothirds of its conductivity at atmospheric pressure, and a heating member for operating said thermostatic member and separated therefrom through said receptacle iilling.

2. A switch comprising a hermetically sealed receptacle, contacts disposed inside said receptacle, a thermostatic member for operating said contacts, a receptacle iilling of rareed hydrogen at a pressure less than glow discharge pressure and below critical but above that value at which the heat conductivity of the hydrogen amounts to two-thirds of its conductivity at atmospheric pressure and a heating member for operating said thermostatic member and separated therefrom through said receptacle filling.

3. A switch comprising a hermetically sealed receptacle, contacts disposed inside said receptacle, a thermal responsive member for operating said contacts, and a receptacle filling of inert gas at a pressure less than glow discharge pressure and below critical but above that value at which the heat conductivity of the gas amounts to twothirds'of its conductivity at atmospheric pressure, said receptacle filling being adapted to eiect the transfer of temperature changes to and from the said thermal responsive member.

4. A switch comprising a hermetically sealed receptacle, contacts disposed inside said receptacle, a heat responsive member for operating said contacts, and a receptacle filling comprising hydrogen at a pressurev less than glow discharge pressure and below critical but above that value at which the heat conductivity of the gas amounts to two-thirds of its conductivity at atmospheric pressure, said receptacle filling being adapted to eiect the transfer of temperature changes to and from the said thermal-responsive member.

' AXEL OSVALD APPELBERG.' 

